Hammer firing system for a high speed printer

ABSTRACT

In a high speed printer the hammers are fired by first applying a firing pulse to the hammer and then applying a damping pulse to the hammer as the hammer is rebounding. The damping pulse cancels out the kinetic energy of the hammer and brings it more quickly to rest each time the hammer is fired. The firing circuit comprises a transistor having the hammer coil connected in the collector circuit thereof and a resistance in the emitter circuit thereof. When the hammer is fired the transistor is initially driven into saturation. When the transistor comes out of saturation, negative feedback provided by the resistance in the emitter circuit maintains the firing pulse current constant to the end of the firing pulse.

United States Patent Pear, Jr. et al.

[54] HAMMER FIRING SYSTEM FOR A HIGH SPEED PRINTER [72] Inventors:Charles B. Pear, Jr., Greenlawn; Joseph A.

Ross, Fort Salonga, both of NY.

[73] Assignee: Potter Instrument Company, Inc., Plainview, NY.

[22] Filed: June 25, I970 [21] Appl. No; 49,767

"25 PR'NT CLOCKA CONTROL 7-39 REFERENCE VOLTAGE [451 July 25, 1972FOREIGN PATENTS OR APPLICATIONS 1,033,505 6/1966 Great Britain ..307/27OOTHER PUBLICATIONS Millman & Taub, Pulse, Digital and SwitchingWaveforms 1965 pp. 192-195 Primary Examiner-John ZazworskyAttorney-Lane, Aitken, Dunner & Ziems 5 7] ABSTRACT In a high speedprinter the hammers are fired by first applying a firing pulse to thehammer and then applying a damping pulse to the hammer as the hammer isrebounding. The damping pulse cancels out the kinetic energy of thehammer and brings it more quickly to rest each time the hammer is fired.The firing circuit comprises a transistor having the hammer coilconnected in the collector circuit thereof and a resistance in theemitter circuit thereof. When the hammer is fired the transistor isinitially driven into saturation. When the transistor comes out ofsaturation, negative feedback provided by the resistance in the emittercircuit maintains the firing pulse current constant to the end of thefiring pulse.

14 Claims, 7 Drawing Figures HAMMER HAMMER PATENTED L I912 3 3.678347sum 1 or 3 n /25 LOCKA HAMMER HAMMER HAMMER PRINT CLOCK I CONTROLeENERATpR )CLOCK B Is I I 39 REFERENCE VOLTAG E I 20 I CLOCK 8. H FM H EPRINT I I I I CONTROL I I 28 I ENABLING SIGNAL I I I I I 30 I I IPULSESW TO HAMMER] I FIRING I CIRCUIT 2 FIRING 4 FlRlNG I PERIOD I IPERIOD I INVENTORS CHARLES B. P, EAR,Jr. a JOSEPH A, Ross FIG. 2

PATENTEDJUL25|912 3.678347 sum 2 or 3 NO RMAL TRANSI EN'T BEH AV IO RDISPLACEMENT 7| R T P T Es OSI ION L v TIME FIRE PULSE:%V73

TFI'E IMPROVED TRANSIENT BEHAVIOR DISPLACEMENT 7s F/ G. 4. to n TIMEDAMPING PULSE FIRE PULSE Tl E HAMMER HAMMER HAMMER-VB I! r A HAMMER 9|FIRING PRINT 4; CIRCUIT CONTROL I MMv DELAY MMv INVENTOR-S F/ G 5.

CHARLES B. PEAR,Jr.8 JOSEPH A. Ross PATENTEDJULZS m2 3I678I847 I saw aor 3 TONE WHEEL PULSES I I I I I I i I I 1 Ema L I m m MMv 97 i l MMVIOSI: [1:

. I I PULSES TO I l HAMMER I l 09 I I FIRING l CIRCUIT I:

ll l2 HAMMER HAMMER HAMMEEL 1 m l I I AMME 29 9 I09 FIRING I I mvERfonPRINT I CONTROL 25 MMv 0ELAY MMv 97 (I03 los INVENTORS F l G. 7. CHARLESB. PEAR,Jr.8

JOSEPH A. ROSS RNEYS BACKGROUND OF THE INVENTION This invention relatesto pulse actuated electromagnetic devices and more particularly to ahammer with an improved firing circuit for high speed printers.

In high-speed, impact printers, the repetition rate at which the hammersoperate must be quite high to enable the printer to print at a highrate. This is particularly true in a printer of the type in which thehammers are controlled in a manner to print the dots in patterns tocompose the selected alphanumeric characters. Accordingly, each hammermust be actuated many times to print each character. In order to obtainuniform printing and prevent what is known in the art as ghosting, it ispreferable that the hammer be actuated from rest position or from near arest position. Thus, it is desirable to bring the hammer to rest asquickly as possible following each actuatron.

The present invention provides a circuit for firing the hammer in amanner to very quickly bring it to rest following each actuation. Inaddition, the hammer firing circuit of this invention provides stableoperation of the hammers despite impedance changes in the operating coildue to temperature changes and the like.

SUMMARY OF THE INVENTION In accordance with the present invention apulse actuated electromagnetic hammer for a high speed printer isenergized by a circuit which first apples an actuating pulse to thehammer operating coil and then applies a properly timed second pulsewith a polarity to oppose the residual kinetic energy in the device andwith an impulse content to approximately cancel the residual kineticenergy. The second pulse is applied as the hammer is rebounding afterstriking the print forming member and has the same polarity as theactuating or firing pulse. The second pulse, which is referred to hereinas a damping pulse, cancels the kinetic energy of the hammer and quicklybrings the hammer to rest. The hammer operating coil is connected in thecollector circuit of a transistor which has an emitter feedback resistorto apply negative feedback to the transistor. In applying a pulse to thehammer coil, the transistor is initially driven to saturation andprovides a steeply rising current pulse to the coil. When the transistorcomes out of saturation, the negative feedback maintains a constantcurrent through the coil until transistor is cut ofi, thus permittingthe energy content of both the firing and damping pulse to be controlledby controlling the duration of the firing and damping pulsesrespectively.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustrates ahigh speed printer with an improved hammer firing system in accordancewith the present application.

FIG. 2 illustrates some wave forms which are utilized in the system ofFIG. 1.

FIG. 3 illustrates the transient behavior of a hammer or other pulseactuated electromagnetic device without the present invention.

FIG. 4 illustrates the improved transient behavior with the presentinvention.

FIG. 5 illustrates an alternative embodiment of a hammer firing systemin a high speed printer in accordance with the present invention.

FIG. 6 illustrates some wave forms used in the system of FIG. 5; and

FIG. 7 illustrates another alternative embodiment of a hammer firingsystem in a high speed printer in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The printerschematically illustrated in FIG. I comprises a drum 1] which carries araised helical bar 12 and hammers I3 for printing selected alphanumericcharacters. For purposes of simplicity the circuit driving only onehammer I3 is shown and described. The remaining hammers l3 distributedalong the length of the drum have similar driving circuits. The drum IIis driven by a motor not shown, and drives a tone wheel I5 whichproduces a series of output pulses from a transducer I7. The charactersare selected to be printed by timing the firing of the hammer 13relative to the position of the drum 1] as it rotates. Usually a paperweb and an ink or carbon ribbon are disposed between the drum 11 and adot is printed each time a hammer fires and pushes the paper and ribbonagainst the helical bar. By firing the hammers 13 at the proper times,the dots will be printed in patterns to compose selected alphanumericcharacters. Because a hammer must be fired several times to print acharacter, the present invention is particularly useful in this kind ofprinter.

Pulses produced by the transducer 17 are applied to a clock generator 19which produces two synchronized output clock pulse trains, clock train Aand clock train B illustrated in FIG. 2 as wave forms 20 and 22. Asshown in FIG. 2, each pulse of clock pulse train B is producedimmediately before the next succeeding pulse of clock pulse train A. Inthe system of the present invention, the pulses of clock train A definehammer firing periods which start and end at the leading edges ofadjacent pulses in clock train A. The hammer 13 can be fired once duringeach firing period. Thus, the hammer 13 can be fired at the same rate atwhich the pulses of the clock train A are produced.

The pulses of clock train A are applied to an AND gate 21 and the pulsesof clock train B are applied to an AND gate 23. As will be describedbelow, the hammer 13 is fired in response to pulses of clock train A andthen is clamped in response to the pulses of clock train B. If thehammer is to be fired in a selected firing period, print control 25 willapply an enabling signal to both of the gates 21 and 23 for a periodlong enough for both the pulse from the clock train A and the followingpulse from clock train B to pass through the gates 21 and 23. Thesepulses upon passing through the gates 21 and 23 are applied through anOR gate 26 to a switch 27 in the hammer firing circuit 29. If the hammer13 is not to be fired during a firing period then the print control 25will not enable the gates 21 and 23 during the firing period. If thehammer I3 is to be fired in two successive periods, the enabling signalfrom the print control 25 will be applied continuously over both firingperiods. The print control signal is synchronized with the clock pulsesby means of pulses applied thereto from clock train A. An exemplarycontrol enabling signal is illustrated in FIG. 2 as wave form 28,showing the shape of the print control signal when the hammer is to befired during a first, third, and fourth firing period of a sequence offour firing periods but not during the second firing period of thesequence. As shown in FIG. 2, the enabling signal is applied during thefirst period and continuously throughout the entire third and fourthperiods but is not applied during the second period. As a result, duringthe first firing period a pulse from clock train A and a pulse fromclock train B will pass through the OR gate 26, pulses from both clocktrains will also pass through the OR gate 26 during the third and fourthfiring periods but will not pass through the OR gate 26 during thesecond firing period. The resulting pulse train applied through the ORgate 26 to the hammer firing circuit 29 is illustrated in FIG. 2 as waveform 30. In response to each pulse applied thereto through the OR gate26, the hammer firing circuit 29 will apply a pulse of is applied to thehammer. The time of occurrence of the firing pulses and damping pulseswill be substantially simultaneous with the pulses of clock trains A andB respectively. In response to the firing pulse, the hammer 13 willstrike the drum I1 and print. As the hammer is rebounding from the drum,the damping pulse is applied and cancels out the kinetic energy ofthehammer. As illustrated in FIG. 2, the clock trains A and B are arrangedso that the damping pulse occurs as near as possible to the time of thefiring pulse in the next firing period without causing hammer jitter.The firing pulse is timed to terminate before the hammer strikes thedrum. Preferably, the duration of the firing pulse is long so that apulse of relatively low amplitude can be used to avoid excessive heatingand achieve the desired hammer velocity.

FIG. 3 illustrates the normal transient behavior of a hammer in a priorart firing system. The curve 71 shows the displacement of the hammerwith time in response to a firing pulse 73 applied at t At time t thehammer strikes the drum and rebounds as is represented by the change indirection of the curve 71. The hammer rebounds to its rest position andmechanically oscillates about the rest position over a significant timeperiod as shown in FIG. 3. In the system of the present invention asshown in FIG. 4 following the firing pulse 73, a damping pulse 74 isapplied to the coil as the hammer approaches its rest position, asrepresented by curve 76. This pulse opposes the kinetic energy of thehammer and effectively cancels most of it out, greatly reducing theamount of mechanical oscillation of the hammer about the rest positionand thereby bringing the hammer much more quickly to rest. Preferably,the impulse content of the damping pulse 75 is related to the impulsecontent ofthe firing pulse in the same proportion as the reboundvelocity ofthe hammer is to the impact velocity. The term impulsecontent" refers to the energy of an applied pulse and is proportional tothe pulse width multiplied by the pulse current amplitude. For constantcurrent pulses, the ratio of the firing and damping pulse widths will beequal to the ratio of the impact and rebound velocities. It will beapparent that the impulse content of the damping pulse, in other wordsthe amount of damping required, is a function of the rebound velocity.Obviously the impulse content of the damping pulse necessarily isinsufiicient to cause the hammer to restrike the drum.

When the electronic switch 27 receives a pulse from the OR gate 26, itcloses and applies a positive voltage from a terminal 3] through aresistor 33 to the base of an NPN transistor 35. The base of thetransistor 35 is connected through a clamping diode 37 to a positivereference voltage level applied at a terminal 39. The collector of thetransistor 35 is connected through a resistor 41 to a positive source of28 volts applied to a terminal 43. The emitter of the transistor 35 isconnected through a kilohm resistor 45 to a minus source of 5 voltsapplied to a terminal 47. The emitter of the transistor 35 is alsoconnected directly to the base of an NPN transistor 49, the emitter ofwhich is connected to ground through a 3 ohm resistor 51 and thecollector of which is connected to the coil of the hammer 13 in seriesto a source of positive voltage ap plied to a terminal 55. The coil ofthe hammer 13 is shunted by a diode 57 and a resistor 59 connected inseries.

When a pulse from either the clock train A or the clock train B isapplied to the switch 27, the switch 27 closes and applies the voltageapplied to the terminal 31 to the base of the transistor 35. This causesthe transistor 35 to conduct which in turn causes the transistor 49 toconduct. As a result, current is applied from the source 55 through thehammer coil and the transistor 49 connected in series. When the inputpulse applied to switch 27 ceases, the transistor 35 will be cut offwhich in turn will cut off the transistor 49 and current flowing throughthe coil will be terminated. The diode 57 and the resistor 59 provide acurrent path to dissipate the energy inductively stored in the coil whenthe transistor 49 is cut off. The resistor 51 provided in the emittercircuit of transistor 49 providesa negative feedback signal to thetransistor 49 which decreases the time it takes the hammer to get up tospeed,

decreases the power dissipation in the hammer, reduces the power supplyvoltage regulation requirements as compared with the systems of theprior art, and minimizes the effects of changes in the impedance of coil13 due to changes in temperature. When a pulse is first applied to theswitch 27 causing the transistor 35 to turn on, the transistor 49initially saturates and the current starts rising up rapidly towards alarge value due to the low resistance in the circuit. In the initialstages of the current buildup the effect of this resistance, that is theinternal coil resistance and the resistance of the resistor 51, isnegligible and the slope of the curve is essentially the voltage dividedby the inductance of the coil of the hammer 13. When the current reachesthe desired value, the transistor 49 comes out of saturation and thecurrent is maintained constant due to the feedback provided by theresistor 51. It should be noted that owing to the fact of negativefeedback of the hammer driver circuit the damping pulse is a constantcurrent pulse and its energy can be conveniently varied by varying thewidth of the pulse in clock train B.

FIG. 5 illustrates an alternative embodiment of the system of thepresent invention. In this system the print control 25 synchronized bytone wheel pulses from the transducer 17 applies a pulse to an AND gate91 each time the hammer is to be fired. FIG. 6 shows an example of thesepulses as wave form 93 and shows how they relate in time to the tonewheel pulses represented by wave form 95. The pulses 93 shown in FIG. 6are intended to cause firing of the hammer 13 in the first, third andfourth firing periods represented in FIG. 6. The tone wheel pulses whichdefine the boundaries of the firing periods are also applied to the gate91. The print control pulses 93 are timed to occur simultaneously withthe tone wheel pulses by means of the synchronization derived from thetone wheel pulses applied to the print control 25. The print controlpulses 93 to insure simultaneity are wider than the tone wheel pulses,each starting before and ending after the corresponding tone wheelpulse. When the gate 91 is enabled by a pulse from the print control 25,the tone wheel pulse occuring at the time of this pulse will passthrough the gate 91 and trigger a monostable multivibrator 97. Thus, inthe example illustrated in FIG. 6, the tone wheel pulses will passthrough the gate 91 at the start of the first, third and fourth firingperiods. As a result, the monostable multivibrator 97 will produce asquare wave output pulse at the start of the first, third and fourthfiring periods as represented by the wave form 99 in FIG. 6. Thesepulses are designed to have a length equal to the length of the firingpulses to be applied to the hammer 13. The output pulses produced by themonostable multivibrator 97 are applied through an OR gate I01 to thehammer firing circuit 29 in exactly the same manner that the pulsespassing through the OR gate 26 in FIG. I are applied to the hammerfiring circuit 29 in the embodiment of FIG. 1. As a result, the hammerfiring circuit 29 in response to each output pulse produced by themonostable multivibrator 97 will apply a firing pulse to the hammer l3and cause it to strike the drum II. The output pulse produced by themonostable multivibrator 97 is also applied through a delay circuit 103to a second monostable multivibrator to trigger the second monostablemultivibrator 105. As a result the monostable multivibrator 105 will betriggered after a delay and produce an output pulse in response to eachoutput pulse produced by the monostable multivibrator 97. These pulsesare illustrated in FIG. 6 by the wave form I07. As shown in FIG. 6,pulses produced by the monostable multivibrator 105 are substantiallysmaller in width than the pulses produced by the monostablemultivibrator 97 and are selected to have a width equal to the width ofthe damping pulses to be applied to the hammer 13 following each firingpulse applied to the hammer 13. The output pulses produced by themonostable multivibrator 105 are applied through the OR gate 101 to thehammer firing circuit 29 and cause the hammer firing circuit 29 to applya pulse of the corresponding width to the hammer 13. Thus following eachhammer firing pulse applied to the hammer 13, a damping pulse of thedesired width is applied. The delay provided by the circuit 103 isselected so that the damping pulse is applied just before the nextfiring pulse would be applied if a firing pulse is to be applied in thenext firing period. Wave form 109 illustrates the pulse wave formapplied to the hammer firing circuit through the gate 101 and thusrepresents the timing of the firing pulses and damping pulses applied tothe hammer 13.

In the embodiments described above, the damping pulses are applied tothe hammer after the hammer has cleared the medium on which the printingis carried out. To further increase the speed of operation, the dampingpulse may be started before the hammer is completely clear of theprinting medium. This will tend to smear the printing when the hammer isfired in successive print periods. This problem can be avoided byeliminating the damping pulse in those instances when the hammer is tobe fired in the next succeeding firing period.

FIG. 7 illustrates a system for carrying out this alternative in whichthe damping pulses are eliminated when a firing pulse is to be appliedin the succeeding firing period. The system of FIG. 6 is just like thatof FIG. 5 except that the output of the monostable multivibrator 105 isapplied through an AND gate 109 to the OR gate 101. The AND gate 109will be enabled by a signal derived from the output of the print control25 through an inverter 113. The inverter 113 will enable the gate 109only when the output of the print control 25 is not producing an outputpulse. Because of the timing of the pulses produced by the print control25, the gate 109 will be enabled when a damping pulse is produced onlyif no firing pulse is to be applied to the hammer 13 in the succeedingfiring period. Thus, in the system of FIG. 7, the damping pulses can beapplied before the hammer clears the medium on which the printing isbeing carried out. It should be noted that the width of the dampingpulse may be varied, if desired, in accordance with whether or not thehammer was just previously fired. Such a system may be desirable owingto the fact that the velocity and certain second order effects such ashammer oscillation are a function of the just previous firing history ofthe hammer. in yet another embodiment of the invention, the actualvelocity of the hammer may be sensed, and the width of the damping pulsedetermined by this actual width in a closed loop system. Conveniently,the actual hammer velocity can be sensed by means ofa coil attached tothe hammer blade and in the field of the hammer magnet.

The above-described systems greatly reduce mechanical oscillation of thehammers after they are fired and thus enable the hammers to be fired ata much higher rate than the systems of the prior art. As pointed outabove, the principles of the invention are applicable to other pulseactuated electromechanical devices in addition to hammers for high speedprinters. The invention will enable such devices to be operated inresponse to pulses at a higher rate in the same manner that it enablesthe hammers of the high speed printer to be operated at a higher rate.The above-described systems are the preferred embodiments of theinvention and many modifications may be made thereto without departingfrom the spirit and scope ofthe invention.

What is claimed is:

1. in a high speed printer having a print member and at least one pulseresponsive electromagnetically operated hammer, said hammer being firedto strike said print member to thereby print in response to firingpulses applied thereto, the improve ment comprising means selectivelyoperable to apply a firing pulse to said hammer to cause said hammer tostrike against said print member and rebound therefrom and to apply asecond pulse to said hammer following said firing pulse while saidhammer is still moving, said second pulse being of a polarity to opposethe kinetic energy of said hammer and being of insufficient impulsecontent to cause said hammer to strike said print member.

2. In a high speed printer as recited in claim 1 wherein said secondpulse is applied while said hammer is rebounding away 6 from said rintmember.

3. In a igh speed printer as recited in claim 2 wherein the ratio of theimpulse content of said second pulse to that of said firing pulse isapproximately equal to the ratio of the velocity of said hammer uponrebounding from said print member to the velocity ofsaid hammer instriking said print member.

4. In a high speed printer as recited in claim 1 wherein the impulsecontent of said second pulse is such to approximately cancel out thekinetic energy of said hammer when said second pulse is applied to saidhammer.

5. In a high speed printer as recited in claim 1 wherein said firingpulse is terminated before said hammer strikes said print member.

6. in a high speed printer as recited in claim 5 wherein said secondpulse is not applied to said hammer until said hammer has rebounded fromsaid print member and hascleared the medium on which said printing iscarried out.

7. In a high speed printer as recited in claim 1 wherein said means toapply pulses to said hammer comprises an amplifier connected to applypulses to said hammer in response to pulses applied to the input thereofand includes means to apply negative feedback signal to said amplifier.

8. In a high speed printer as recited in claim 7 wherein said amplifiercomprises a transistor, wherein said hammer has an actuating coilconnected in the collector circuit of said transistor, and wherein aresistance is connected in the emitter circuit of said transistor toapply said negative feedback signal to said amplifier.

9. In a high speed printer as recited in claim 3 wherein said means toapply pulses to said hammer comprises an amplifier connected to applypulses to said hammer in response to pulses applied to the inputthereof, said amplifier including negative feedback means formaintaining a constant current amplitude of said pulses applied to saidhammer.

10. In a high speed printer comprising a print member and at least onepulse-responsive, electromagnetically-operated hammer, said hammer beingfired to strike said print member and carry out printing in response tofiring pulses applied thereto, the improvement comprising meansselectively operable to apply a firing pulse to said hammer in each of aseries of successive firing periods and to apply a damping pulse to saidhammer following each firing pulse only if no firing pulse is to beapplied to said hammer in the next succeeding firing period, saiddamping pulse having the same polarity of said firing pulse and beingapplied to said hammer while said hammer is rebounding from the mediumon which the printing is being carried out as a result of the precedingfiring pulse, said damping pulse having insufficient impulse content tocause said hammer to restrike said print member.

11. A combination comprising a pulse responsive electromagneticallyoperated device which operates to drive an output member against a stopin response to an applied pulse and means to apply a first pulse to saiddevice to drive said output member against said stop and to apply asecond pulse to said device following said first pulse applied theretoafter said output member has been driven against said stop while saidoutput member is still moving, said second pulse having a polarity tooppose the kinetic energy of said output member and having insufficientinpulse content to drive said output member against said stop.

12. A combination as recited in claim 13 wherein said second pulse isapplied to said device while said output member is rebounding away fromsaid stop.

13. A combination as recited in claim 14 wherein the ratio of theimpulse content of said second pulse to that of said first pulse isapproximately equal to the ratio of the velocity of said output memberin rebounding from said stop to the velocity of said output member instriking said stop.

14. A combination as recited in claim 13 wherein the impulse content ofsaid pulse is selected to approximately cancel out the kinetic energy ofsaid output member.

1. In a high speed printer having a print member and at least one pulseresponsive electromagnetically operated hammer, said hammer being firedto strike said print member to thereby print in response to firingpulses applied thereto, the improvement comprising means selectivelyoperable to apply a firing pulse to said hammer to cause said hammer tostrike against said print member and rebound therefrom and to apply asecond pulse to said hammer following said firing pulse while saidhammer is still moving, said second pulse being of a polarity to opposethe kinetic energy of said hammer and being of insufficient impulsecontent to cause said hammer to strike said print member.
 2. In a highspeed printer as recited in claim 1 wherein said second pulse is appliedwhile said hammer is rebounding away from said print member.
 3. In ahigh speed printer as recited in claim 2 wherein the ratio of theimpulse content of said second pulse to that of said firing pulse isapproximately equal to the ratio of the velocity of said hammer uponrebounding from said print member to the velocity of said hammer instriking said print member.
 4. In a high speed printer as recited inclaim 1 wherein the impulse content of said second pulse is such toapproximately cancel out the kinetic energy of said hammer when saidsecond pulse is applied to said hammer.
 5. In a high speed printer asrecited in claim 1 wherein said firing pulse is terminated before saidhammer strikes said print member.
 6. In a high speed printer as recitedin claim 5 wherein said second pulse is not applied to said hammer untilsaid hammer has rebounded from said print member and has cleared themedium on which said printing is carried out.
 7. In a high speed printeras recited in claim 1 wherein said means to apply pulses to said hammercomprises an amplifier connected to apply pulses to said hammer inresponse to pulses applied to the input thereof and includes means toapply negative feedback signal to said amplifier.
 8. In a high speedprinter as recited in claim 7 wherein said amplifier comprises atransistor, wherein said hammer has an actuating coil connected in thecollector circuit of said transistor, and wherein a resistance isconnected in the emitter circuit of said transistor to apply saidnegative feedback signal to said amplifier.
 9. In a high speed printeras recited in claim 3 wherein said means to apply pulses to said hammercomprises an amplifier connected to apply pulses to said hammer inresponse to pulses applied to the input theReof, said amplifierincluding negative feedback means for maintaining a constant currentamplitude of said pulses applied to said hammer.
 10. In a high speedprinter comprising a print member and at least one pulse-responsive,electromagnetically-operated hammer, said hammer being fired to strikesaid print member and carry out printing in response to firing pulsesapplied thereto, the improvement comprising means selectively operableto apply a firing pulse to said hammer in each of a series of successivefiring periods and to apply a damping pulse to said hammer followingeach firing pulse only if no firing pulse is to be applied to saidhammer in the next succeeding firing period, said damping pulse havingthe same polarity of said firing pulse and being applied to said hammerwhile said hammer is rebounding from the medium on which the printing isbeing carried out as a result of the preceding firing pulse, saiddamping pulse having insufficient impulse content to cause said hammerto restrike said print member.
 11. A combination comprising a pulseresponsive electromagnetically operated device which operates to drivean output member against a stop in response to an applied pulse andmeans to apply a first pulse to said device to drive said output memberagainst said stop and to apply a second pulse to said device followingsaid first pulse applied thereto after said output member has beendriven against said stop while said output member is still moving, saidsecond pulse having a polarity to oppose the kinetic energy of saidoutput member and having insufficient inpulse content to drive saidoutput member against said stop.
 12. A combination as recited in claim13 wherein said second pulse is applied to said device while said outputmember is rebounding away from said stop.
 13. A combination as recitedin claim 14 wherein the ratio of the impulse content of said secondpulse to that of said first pulse is approximately equal to the ratio ofthe velocity of said output member in rebounding from said stop to thevelocity of said output member in striking said stop.
 14. A combinationas recited in claim 13 wherein the impulse content of said pulse isselected to approximately cancel out the kinetic energy of said outputmember.